Experimental Study On Thermal Properties Of Beishan Granite Under Moderate Temperature Conditions

The safe disposal of high-level radioactive waste(HLW)is the key to the sustainable operation of nuclear energy.In the process of deep underground disposal of high-level radioactive waste,the radionuclide decay process will release a large amount of heat,causing the surrounding rock to be in a higher temperature field.Temperature will affect the physical and mechanical properties of rocks,thereby affecting the airtightness and mechanical carrying capacity of the repository,reducing the safety of the repository and its storage capacity.Therefore,it is of practical significance to study the effect of temperature on the physical and mechanical properties of rocks within the release temperature range of high-level radioactive waste.In the current research on the influence of temperature on the mechanical properties of rocks,most of the temperature range concerned exceeds the maximum temperature that can be reached at the attenuation of high-level radioactive waste,and there is a lack of refined research in the actual temperature range that can be reached in underground repositories.Therefore,this paper carries out experimental research on the thermal properties of Beishan granite under medium temperature conditions(room temperature ? 300?),which provides certain reference value for related projects.The main research results of this paper are as follows:(1)Acoustic emission monitoring of the whole process of heating and cooling of Beishan granite under different temperature conditions was carried out.By analyzing the evolution characteristics of acoustic emission impact number,energy,value,rise time/amplitude and average frequency value(RA-AF value)in the whole process of heating and cooling,the thermal fracture evolution law of Beishan granite in the whole process of heating and cooling is studied.The results show that: Acoustic emission impact number analysis found that the number of impact signals generated during the heating process of granite was greater than the number of signals generated during the cooling process;acoustic emission energy analysis found that the higher the temperature of the granite,the greater the energy released during the cooling process;The value analysis shows that the value of granite will decrease during the cooling process;the RA-AF value analysis shows that as the temperature increases,the proportion of microcracks in the form of shear fracture in the granite increases during the cooling process.In summary,the number of thermal cracks produced in the heating process of granite is relatively large,the size is small,and the form of cracking is mainly tensile and the energy release is small.However,the number of thermal cracks produced during the cooling process is small and the size is relatively small.In the large and fractured forms,the proportion of shear fracture increases and the energy release is greater.With the increase of heating temperature,the number of thermal fractures produced by granite in the cooling stage gradually increases.(2)The uniaxial compression test and Brazilian split test of Beishan granite under temperature conditions were carried out,supplemented by acoustic emission monitoring,to study its mechanical properties and acoustic emission evolution characteristics under the combined action of temperature and load.The results show that during uniaxial compression under temperature conditions,with the increase of temperature,the peak strength of granite first increases and then decreases,and the peak strength reaches the maximum at 150?.Acoustic emission energy analysis shows that as the temperature increases,the acoustic emission energy release changes from a sudden increase in nodes to a linear release.The Brazilian split test under temperature conditions shows that with the increase of temperature,the tensile strength of Beishan granite first increases and then decreases.The analysis of its fracture characteristics shows that temperature has a certain effect on its cleavage path;the statistics of the fractures between mineral components show that temperature has a significant weakening effect on the cementation between minerals,and the high cementing strength of quartz-quartz,feldspar-Feldspar is obviously affected.As the temperature increases,more cracks begin to develop in these two types of mineral combinations.(3)A cyclic loading and unloading test of Beishan granite under temperature conditions was carried out,supplemented by acoustic emission monitoring.Through the analysis of acoustic emission energy,loading response ratio,accelerated energy release fitting parameters,Felicity ratio,etc.,the cyclic loading and unloading process of granite was studied.The mechanical properties and the evolution characteristics of acoustic emission in.The results show that in the previous cycle of peak failure,the area of ??the granite hysteresis ring increased significantly,and with the increase of temperature,the fracture of granite changed from brittle fracture to ductile fracture.Acoustic emission energy analysis shows that the release of acoustic emission energy changes from a sudden increase mode under low temperature conditions to a gradual increase mode under high temperature conditions.The analysis of loading response ratio shows that even in the elastic stage,the acoustic emission loading response ratio is still greater than 1,indicating that under the combined action of temperature and load,the internal microcracks of granite are steadily developing in the elastic stage.Accelerated energy release studies have shown that the energy release of the cyclic loading and unloading test under granite temperature changes from an accelerated release mode at a low temperature to a linear release mode at a high temperature,indicating that the higher the temperature,the more complex the internal system of the granite.The Felicity ratio study shows that the damage strength of granite is in the range of 0.55 to 0.75 peak strength,which is smaller than the damage strength at room temperature.(4)Nuclear magnetic resonance tests were carried out on Beishan granite treated at different temperatures.The results showed that the granite pore structure showed "bimodal characteristics",and the pore closure phenomenon appeared after treatment at 150?,and the pore diameter and pore volume of the large pore section There is a significant decrease,which is manifested as a phenomenon of "heat healing",which is closely related to the increase in strength under this temperature condition.Through FISH language programming,considering the influence of temperature on the strength and deformation parameters of granite,a thermal-mechanical coupled numerical simulation of the typical KBS-3V high-level radioactive waste disposal model was carried out within 100 years of disposal,and the changes in elastic modulus and strength under varying temperature conditions during the disposal process were discussed.On the influence of the stress-strain field,it is found that the elastic modulus changes with the temperature will significantly change the time and magnitude of the maximum principal stress of the rock mass.The change of rock mass mechanical strength parameters under variable temperature conditions has an impact on the rock mass strength distribution and strength-stress ratio,as well as the stability of the disposal pit.